Reading differential voltage

Some ADCs in the market allow you to read differential voltage. How is this different from single ended voltage? Single ended voltage is the potential difference between a node and ground. Differential voltage is the potential difference between two nodes. A use case of measuring differential voltage is to measure the current flowing in a circuit by measuring the voltage across a shunt resistor.

You’ve picked the ADC you want to use, in my case LTC2990 and in the datasheet it shows the differential voltage range as -0.3V to +0.3V. But this is not the only thing spec you need to design for! Be sure to check what common mode voltage can the sensor allow on the differential voltage measuring pins. Usually it won’t be more than Vcc (supply voltage of the sensor chip).

Lets take an example. You want to measure the current going into a Li-ion cell with voltage range 2.8V to 4.2V. Say the max current that can be used for charging the cell is 1.5A. You spec your shunt resistor at 0.2 ohm, so that the differential voltage does not exceed 0.3V. Your LTC2990 ADC chip is running on a Vcc supply of 3.3V. Will your design work? The answer is no. It is because the common mode voltage can go upto 4.2V on the differential pins which is greater than the supply voltage (3.3V) of the ADC chip.

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